WASHINGTON — Just days before Election Day, and with voters in many states already going to the polls, the F.B.I. director made a stunning announcement on Friday: Agents had discovered new emails that might be relevant to the completed investigation into Hillary Clinton’s private server, a case that she had seemingly put behind her in July.

Never in recent history has the F.B.I. been so enmeshed in a presidential race. With a vague 166-word statement to Congress, the F.B.I. sent jolts through the campaign, leaving many voters unsure what to make of a case involving national security secrets, a disgraced congressman, racy text messages and a dispute among the country’s top law enforcement officers.

Here’s what we know so far.

What happened on Friday?

The F.B.I. director, James B. Comey, sent a letter to Congress that said agents had uncovered new emails that could be connected to the Clinton investigation. That investigation had examined whether Mrs. Clinton and her aides had mishandled classified information by sending it through Mrs. Clinton’s private email server. The inquiry was completed in July with no charges filed.

Mr. Comey said on Friday that agents would review the new emails to see whether they contained classified information. The letter was sent 11 days before the presidential election, and it set off fierce criticism of Mr. Comey for appearing to meddle in politics.

The F.B.I. director’s letter did not reopen the Clinton inquiry, though some Republicans, including Donald J. Trump, characterized the move that way. Agents could open a new inquiry if they find evidence that the earlier investigation had been impeded or that classified materials had been intentionally mishandled.

Mr. Comey did not say in his letter. But law enforcement officials briefed on the investigation said that agents had discovered the emails on a laptop owned by Anthony D. Weiner, the disgraced former congressman and estranged husband of Mrs. Clinton’s top aide, Huma Abedin.

Last month, the F.B.I. began investigating allegations that Mr. Weiner had exchanged sexually explicit messages with a teenager. On Oct. 3, agents in New York executed a search warrant to obtain Mr. Weiner’s iPhone, an iPad and the laptop. Searching the laptop, they found evidence of a trove of emails similar to ones that had been examined in the Clinton investigation.

Mr. Comey decided last week that agents should examine those emails to determine whether they contained national security information. That requires a court order, and officials said agents had not yet begun reading the emails.

Why does the F.B.I. care if there is classified information in the emails?

Under federal law, mishandling national security information is a crime, one that the F.B.I. is responsible for investigating. In 2015, the bureau began investigating the personal email account that Mrs. Clinton had used exclusively as secretary of state. As part of that investigation, the bureau tried to find every electronic device — phones, tablets, computers — that Mrs. Clinton and her aides used.

Agents could not find many of them, including several of Mrs. Clinton’s cellphones and two iPads. The agents knew that those devices, and others they were not aware of, might someday surface. But they completed the Clinton case because they found no evidence that anyone had intentionally broken the law.

The newly discovered emails may — or may not — provide new information to the F.B.I.

Why did Mr. Comey send the letter?

In July, Mr. Comey told Congress that the Clinton investigation was complete but that if new information came to light, the bureau would examine it. Mr. Comey pledged to be as transparent as he could with Congress about the investigation, and has since made public hundreds of pages of documents related to the inquiry.

According to senior F.B.I. officials, Mr. Comey felt that he would be breaking his pledge of transparency to Congress if he did not reveal the new information from the Weiner case. And he believed that the bureau would be accused of suppressing details to benefit Mrs. Clinton — an accusation that he believed could do lasting damage to the F.B.I.’s credibility.

Who is upset with Mr. Comey for sending the letter?

Many Democrats and even some Republicans have called the letter vague, troubling and unprecedented. Senior officials at the Justice Department urged Mr. Comey not to send the letter, saying it violated the spirit of longstanding policies not to discuss current investigations or do anything that could be seen as meddling in an election.

In the letter, Mr. Comey said that the F.B.I. had yet to determine whether “this material may be significant,” and that he could not predict how long the review would take. Mrs. Clinton’s campaign has pushed Mr. Comey to release more information about the emails.

Her campaign chairman, John D. Podesta, said that “by providing selective information, he has allowed partisans to distort and exaggerate to inflict maximum political damage.”

What does all of this mean for Mrs. Clinton and her campaign?

The short answer is that it is not yet clear. Polling on weekends can be unreliable, so it may be a few days before the effect of the development can be fully assessed. What is evident is that a campaign that has largely been a referendum on Mr. Trump — particularly since the first debate — is now not so clear-cut. The email development will certainly matter, but the question is just how much.

Twenty million people have already voted, and millions more have already determined whom they will support. The country was already politically polarized before this election, and opinions are overwhelmingly cemented about these two household-name nominees.

The email news could matter most in down-ballot races. After being on the defensive for weeks because of Mr. Trump’s behavior, Republican candidates now have a more helpful news media environment in which to make their closing arguments. And Republican voters who are otherwise demoralized may have been given one final nudge to show up to the polls.

What happens now?

In the coming days, the F.B.I. will begin conducting a smaller version of the larger investigation it completed in July. Agents will go through the emails found on the laptop to determine whether they contain classified information.

If so, the bureau will again look at the question of whether anyone intentionally committed a crime. Clinton campaign officials have said that Ms. Abedin gave the authorities all of the electronic devices that she believed had work-related emails on them. Many of the newly discovered messages are likely to be duplicates of others that the F.B.I. has already examined, investigators say. The review will be conducted by the same Washington-based F.B.I. agents who led the investigation into Mrs. Clinton’s emails. F.B.I. agents are all but certain that it will not be completed by Election Day, and believe it will take at least several weeks.

Neither Justice Department officials nor F.B.I. agents say they know what to expect from Mr. Comey over the coming days. Normally, investigations are conducted secretly, but Mr. Comey’s public remarks have opened him up to demands from both campaigns that he make as much information public as possible as soon as it is available.

What’s the worst-case scenario for Mrs. Clinton and her aides?

In July, Mr. Comey announced that the bureau had not found enough evidence to charge anyone with a crime for the mishandling of classified materials on Mrs. Clinton’s server. If the new emails indicate intentional efforts by her or her aides to move such information outside secure government systems, or if they tried to impede the earlier inquiry, the F.B.I. will most likely want to investigate further. But the bureau has just begun the process of combing through the new emails, and officials believe that at least some are duplicates of messages that have already been examined.

How rare is it for the F.B.I. to make a development like this public?

Extremely rare. At times during trials or after cases are closed, the F.B.I. finds new evidence and either discloses it to defense lawyers or reopens a case. An F.B.I. director has never made such a disclosure to Congress so close to a presidential election.

How did Attorney General Loretta Lynch and the Justice Department feel about Mr. Comey’s letter?

Senior Justice Department officials tried to discourage Mr. Comey from sending the letter, saying that it would violate department guidelines that advise against talking about current criminal investigations or being seen as meddling in elections.

They urged Mr. Comey not to do anything before Election Day, and they said he should tell Congress when agents had read the emails, understood whether they were relevant and could put them in context. They stopped short, however, of issuing a direct order prohibiting him from sending the letter.

A version of this article appears in print on , Section A, Page 15 of the New York edition with the headline: 10 Questions (and Answers) About the New Email Trove. Order Reprints | Today’s Paper | Subscribe